A conventionally known material for forming microlenses is polyhydroxystyrene. However, the material has problems as below and there has been room for improvement. When polyhydroxystyrene is used as a material for forming a microlens, the microlens is likely to have a deformed hemispherical shape due to heating at a high temperature, and thus it is difficult to form a microlens having a stable shape. Furthermore, polyhydroxystyrene is likely to be colored by heat treatment and tends to have reduced transparency, and hence the discoloration may be observed during use of the microlens.
Meanwhile, there are disclosed a photoresist composition, a composition for forming an anti-reflective coating, a material for forming a resist protective film for immersion lithography, and the like that contain a maleimide copolymer (Patent Documents 1 to 4).
In order to develop an insulating film for display devices such as a liquid crystal display (LCD) and an organic electroluminescent display device, there is disclosed a method for forming an insulating film for a display device characterized by using a radiation-sensitive resin composition that contains an alkali-soluble copolymer composed of indene, maleimide, and an N-substituted maleimide, a 1,2-naphthoquinone diazide compound, and a cross-linking agent (Patent Document 5).
In order to be suitably used for various applications that require characteristics of excellent compatibility with a polyfunctional (meth)acrylate and good alkali-solubility, there is disclosed a maleimide copolymer characterized by including a monomer structural unit derived from unsubstituted maleimides, a monomer structural unit derived from styrenes, and a monomer structural unit derived from (meth)acrylic acids (Patent Document 6).
However, these related art documents do not intend to provide applications as a photosensitive resin composition for a microlens as well as do not suggest specific means and effects of the application of an unsubstituted maleimide or a copolymer including unsubstituted maleimide and N-substituted maleimide unit structures to a photosensitive resin composition for a microlens.
There is also disclosed a radiation-sensitive resin composition characterized by including [A] a polymer of (a1) an unsaturated carboxylic acid and/or an unsaturated carboxylic acid anhydride, (a2) an epoxy group-containing unsaturated compound, (a3) a maleimide monomer, and (a4) another olefinic unsaturated compound and [B] a 1,2-quinonediazide compound (Patent Document 7). The document describes that the disclosed radiation-sensitive resin composition can achieve high radiation sensitivity and can easily form a patterned thin film excellent in solvent resistance, heat resistance, transparency, and heat discoloration resistance. However, the document does not suggest the shape and the heat resistance of a pattern formed from the composition. Furthermore, the document does not describe an unsubstituted maleimide as the maleimide monomer.